Heat transfer for a hard-drive pre-amp

ABSTRACT

A substrate for mounting a preamp chip thereupon, fabricated using a stiffener layer made of a conductive material; an insulating layer provided over the circuitry area of the substrate; a circuitry made of a conductive material provided over the insulating layer; and a flap which is an extension of the stiffener layer having no insulating layer provided thereupon. The flap is fabricated to fold over the preamp chip to remove heat therefrom.

CROSS-REFERENCES TO RELATED PATENT APPLICATIONS

This is a Divisional Application of U.S. application Ser. No. 13/214,120filed Aug. 19, 2011, which is a Continuation Application of applicationSer. No. 11/548,681, filed Oct. 11, 2006 the disclosures of which areincorporated herein in their entirety by reference.

BACKGROUND

1. Field of the Invention

The subject invention relates to hard drives and, more particularly forcontrolling the heat generated by the hard disk drive heads preamp.

2. Related Art

FIG. 1 a depicts a prior art hard drive 100 with the cover removed,while FIG. 1 b depicts an enlarged image of the preamp area. The harddisk 100 uses rotating platters (disks) 110 to store data. Each platteris rotated by a spindle (not shown) and has a smooth magnetic surface onwhich digital data is stored. Information is written to the disk byapplying a magnetic field from a read-write head (not shown) that isattached to an actuator arm 120. For reading, the read-write headdetects the magnetic flux emanating from the magnetic bits that werewritten onto the platter. Since the signals from the read/write head isvery faint, a preamp 130 is provided in close proximity to the head. Thepreamp 130 is a chip that is mounted on a substrate 140. The substrate140 is mounted onto a carrier plate 150, that connects to the actuatorarm assembly 120. The flexible circuit loop 160 is connected to thesubstrate 140, to transfer signals between the preamp 130 and theassociated electronics (not shown). The associated electronics controlthe movement of the actuator and the rotation of the disk, and performreads and writes on demand from the disk controller.

FIG. 2 depicts a prior art preamp sub-assembly, showing a carrier plate250, upon which the substrate 240 is mounted. The preamp 230 is attachedto the substrate 240 and makes electrical connections to tap points onthe substrate 240. As shown in the cross-section inside the broken-linecallout, the substrate is generally made of a stainless steel oraluminum backing, generally referred to as a stiffener, 215, aninsulating polyimide layer 225, and copper conducting contacts and lines235. The “legs” of the preamp chip 230 (or bumps in case of a flip chip)are soldered to the copper contacts 235. In the case depicted, substrate240, having its own stiffener 215, folds back a top carrier plate 250.Carrier plate 250 and stiffener 215 can be made from a common metallayer. Alternate designs integrate the function of the carrier plateinto the stiffener, eliminating the need for the carrier plate. Thesubstrate is generally made using a sheet of stiffener material, uponwhich several substrates are formed, as illustrated in FIG. 3. Asdepicted in FIG. 3, a sheet of stiffener material, such as stainlesssteel or aluminum, 315, serves as a starting material for fabricatingthe substrate 345. For each substrate 345, a polyimide layer 325 isdeposited on top of the stiffener 315 to serve as an electricalinsulator. On top of the polyimide various conductive elements 335 aredeposited to form contacts and transmission lines. The fabrication ofthese layers is done using conventional photolithography techniques.Both subtractive and additive flexible circuit fabrication processes arecommonly employed in hard disk drives. To maximize the available realestate, the substrates 345 are fabricated so as to “nest” with eachother, and after the fabrication is completed the substrates 345 are cutout of the stiffener sheet 315.

As the physical size of the hard drive decrease, the heat generated bythe preamp affects performance and reliability of the hard drive.

SUMMARY

The present invention has been made by observing a problem in the priorart, in that the heat generated by the preamp is not readily dissipated.While the carrier arm 250 can be used as a heat sink, the inventors ofthe subject application have discovered that little heat passes from thepreamp 230 to the carrier arm 250. The inventors have postulated thatthe reason for the low heat transfer is that the polyimide layer 225 ofthe substrate 240 acts as a heat barrier. Notably, polyimide has athermal conductivity of 0.12 W/m-K, which is thermally insulative.Additionally, conductive pads 235 provide a very limited conductive heatrelease means, and suffer as well from the thermal isolation of thepolyimide layer. Accordingly, the inventors have invented schemes tobetter remove heat from the preamp by providing a thermal conduit fromthe top of the preamp to the carrier arm.

According to an aspect of the invention, a substrate for mounting apreamp chip thereupon is provided, comprising a stiffener layer made offirst conductive material; an insulating layer provided over circuitryarea of the substrate; a circuitry of a second conductive materialprovided over the insulating layer; and a flap comprising an extensionof the stiffener layer having no insulating layer provided thereupon,and wherein the flap is fabricated to fold over the preamp chip.According to one aspect, the first conductive material comprisesstainless steel or aluminum. According to another aspect, the secondconductive material comprises copper. The flap may comprise fins. Theflap may also comprise cutout configured for injective adhesivethereupon.

According to another aspect of the invention, an actuator assembly for ahard disk drive is provided, comprising: an actuator arm; a circuitrysubstrate mounted onto the arm; a preamp chip mounted onto the circuitrysubstrate; and, wherein the substrate comprises a flap folded over topof the preamp ship. The substrate may comprises: a stiffener layer madeof first conductive material; an insulating layer provided overcircuitry area of the substrate; a circuitry of a second conductivematerial provided over the insulating layer; and, wherein the flapcomprises an extension of the stiffener layer having no insulating layerprovided thereupon. An adhesive may be provided between the preamp chipand the flap. The flap may comprise a cutout for an adhesive injectedvia the cutout. The adhesive may comprise a heat conducting epoxy. Theflap may comprise fins.

According to yet another aspect of the invention, a method formanufacturing a substrate for supporting an integrated circuit chipthereupon is provided, comprising: providing a sheet of stiffenercomprising a first conductive material; providing an insulating layer ondefined sections of the stiffener, each section defining a circuitryarea of one substrate; providing contacts on the insulating layer, thecontacts made of a second conductive material; and, cutting eachsubstrate out of the sheet according to a designed outline, the designedoutlined comprising the circuitry area and a flap, the flap comprising asection of the sheet of stiffener having no insulating layer thereupon.The method may also comprise cutting a cutout in the flap.

According to a further aspect of the invention, a method formanufacturing a preamp assembly for a hard drive is provided,comprising: providing a substrate, the substrate comprising a stiffenerconductive layer, an insulating layer provided on the stiffener anddefining a circuitry area, and a plurality of contacts provided on theinsulating layer, and a flap comprising a section of the stiffenerhaving no insulating layer thereupon; mounting the preamp on thecircuitry area of the substrate so as to form electrical connection toat least some of the contacts; and folding the flap over the preamp. Themethod may further comprise injective adhesive between the preamp andthe flap. The flap may comprise a cutout and the method may furthercomprise injecting adhesive onto the cutout.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and features of the invention would be apparent from thedetailed description, which is made with reference to the followingdrawings. It should be appreciated that the detailed description and thedrawings provide various non-limiting examples of various embodiments ofthe invention, which is defined by the appended claims.

FIG. 1 a depicts a prior art hard drive 100 with the cover removed,while FIG. 1 b depicts an enlarged image of the preamp area.

FIG. 2 depicts a prior art preamp subassembly.

FIG. 3 depicts a sheet of stiffener having several substrates fabricatedthereupon.

FIG. 4 depicts a preamp subassembly with a substrate according to anembodiment of the invention.

FIG. 5 depicts a preamp subassembly with a substrate according toanother embodiment of the invention.

FIG. 6 illustrates a sheet of stiffener having several substratesfabricated thereupon according to an embodiment of the invention.

FIG. 7 a depicts the resulting temperature distribution in the preampfor the prior art assembly without the flap, while FIG. 7 b depicts thetemperature distribution in the chip with the flap according to theembodiment of FIG. 5.

FIG. 8 a depicts a finite element simulation run of the model with theflap, but without the epoxy, while FIG. 8 b depicts a run of the modelwith the flap and epoxy.

FIG. 9 depicts another embodiment of the invention.

FIG. 10 is a plot depicting simulated temperature reductions due to theflap design versus heat rise in the preamp chip.

DETAILED DESCRIPTION

FIG. 4 depicts a preamp subassembly showing carrier plate 450 with asubstrate 440 according to an embodiment of the invention. Notably, aflap 465 of stiffener material has been added to the fabrication of thesubstrate 440. The flap 465 is folded over the preamp 430, so as toremove heat from the top of the preamp 430. The heat removed isconducted by the flap 465 to stiffener 415, then to carrier plate 450and to the actuator arm assembly (not shown), which acts as theconduction heat sink.

That is, as can be understood from FIG. 2, the stiffener layer 215 ofthe substrate 240 is in physical contact with the carrier plate 250.However, as noted above in the prior art, the heat from the preamp 230is not conducted to the carrier plate 250 because the polyimide layer225 of the substrate 240 acts as a barrier for heat conduction. On theother hand, the flap 465 of the embodiment of FIG. 4 has no polyimidedeposited thereupon. Consequently, heat from the top of the preamp 430can be easily conducted to the flap 465. Since the flap 465 isfabricated as an integral part of the substrate 440, the heat from theflap 465 is easily conducted onto the carrier plate 450. Convective heattransfer also takes place from the exposed surface area of the flap 465to ambient air.

To further improve heat conductance to the flap, optionally a conductiveadhesive 475 can be provided between the preamp 430 and the flap 465, asis illustrated by the broken-line callout 475. In practice, an air gapbetween flap 465 and 430 may exist due to geometric tolerances andforming uncertainties, so the conductive adhesive 475 is useful infilling that poor conductive path. On the other hand, to ease assemblyof the preamp and substrate, in FIG. 5 a cut-out 585 has been made inthe flap 565. In this embodiment, once the preamp 530 is attached to thesubstrate 540, the flap 565 is folded over the preamp, as shown in FIG.5. Then a conductive adhesive is injected into the cutout 585, so as tospread under the flap 565 and onto the preamp 530, as is illustrated bybroken-line callout 575.

FIG. 6 illustrates a sheet of stiffener having several substratesfabricated thereupon according to an embodiment of the invention. InFIG. 6, the starting material is a sheet of stiffener material, such asstainless steel 615. Several substrates 645 are fabricated on sheet 615in a nested arrangement. Each substrate 645 has a “circuit” region,defined by the polyimide layer 625 and shown in solid line, and a flap685, which is a differently processed area of the stainless steel. Thatis, flap 685 is not covered with a polyimide, but is rather barestiffener material. When the substrate is cut out of the sheet material,the cut is made so that the flap is an integral part of the substrate.This ensures that heat conducted onto the flap would be immediatelyconducted to the entire stiffener layer of the substrate. Since thestiffener contacts the carrier plate, the heat would be conducted to itand to the actuator arm assembly, which acts as a heat sink.

During assembly, preamp 530 is attached to the substrate 645, substrate645 is folded along dash-dotted line 696 so as to mate carrier plate 550and stiffener 515, as shown in FIG. 5. Incidentally, as shown in FIG. 5,in this embodiment at the area of fold 696 there is no stiffenermaterial, but rather only a polyimide layer. The flap 685 is then foldedalong dotted line 698 over the preamp 530. Then, when a cutout is used,conductive adhesive is injected into cutout 585. When no cutout isprovided, the adhesive may be injected from the sides, or injected priorto folding the flap over the preamp. One type of adhesive that issuitable for use with the embodiments described herein is TIGA HTR-815epoxy, available from Resin Technology Group of South Easton, Mass. Thisepoxy has thermal conductivity of 1.15 W/m-K, which is an order ofmagnitude higher than the polyimide.

The embodiment depicted in FIG. 5 has been entered into a freeconvection thermal finite element simulation (hereon referred to asmodel) assuming a 25° C. ambient temperature and a fixed self heatgeneration magnitude in the preamp chip volume. The model was runassuming a fixed film coefficient for all exposed surfaces of the chipand surrounding sub-assembly bodies, allowing heat transfer to theambient air by convection. For the first run, the exposed surfaces wereset to have a film coefficient of 1 e⁻⁴W/mm²-K and the preamp self heatgeneration magnitude assumed was 0.05 W/mm³, or 250 mW. FIG. 7 a depictsthe resulting temperature distribution in the preamp for the prior artassembly without the flap, while FIG. 7 b depicts the temperaturedistribution in the chip with the flap according to the embodiment ofFIG. 5. For illustration purposes, the surrounding structures are hiddenin FIGS. 7 a and 7 b. The maximum temperature observed in FIG. 7 a was60.1° C., while for FIG. 7 b with the flap it was 54.7° C. Additionally,without the flap, a large area of high temperature was observed on thepreamp with the gradient increasing towards the center of the preamp,while with the flap, the center of the preamp was cooler than the edges,tending to show that heat is conducted to the flap via the epoxy.Therefore, it is believed that large coverage of epoxy over the preampwould lead to improved results.

The model was also run with the exposed surfaces set to have filmcoefficient of 2.0 e⁴W/mm²-K and the same heat generation magnitude. Forthis case the maximum observed preamp temperature was 52.0° C. withoutthe flap and 47.1° C. with the flap. This tends to show that even whenimproved convection to ambient air is present, the flap still providesthe benefits of heat removal from the chip.

FIG. 8 a depicts a run of the model in FIG. 5 with the flap, but withoutthe epoxy, while FIG. 8 b depicts a run of the model with the flap andepoxy. As can be seen from FIG. 8 a, there is poor thermal conductivitybetween the preamp and the flap due to an air gap placed intentionallybetween them, when no epoxy is present. This exemplifies the prior artconfiguration to an extent, because heat is not being removed from thetop of the chip. On the other hand, the center of the preamp is coolerat the center when the epoxy is added. Both the maximum chip temperatureand average temperature within the chip volume, are reduced.Consequently, it can be seen that if no epoxy is provided, physicalcontact between the flap and the preamp must be assured, which mayincrease manufacturing tolerances. The epoxy enables relaxing thesetolerances and ease manufacturing.

Another embodiment is depicted in FIG. 9. In FIG. 9, heat removal fromthe preamp 930 is enhanced by adding fins 995 to the flap 965. In thismanner, heat is dissipated from the preamp to the flap, and from theflap to the carrier plate by conduction and to the ambient atmospherevia enhanced convection from the fins. Of course, other designs of finscan be made and those shown in FIG. 9 are provided only as one example.

FIG. 10 is a plot depicting expected temperature reduction due to theflap design versus heat rise in the preamp chip. To simulate this trend,the preamp heat generation is varied from 0.05 to 0.2 W/mm³. It isshown, as the observed heat differential rises in the chip, the benefitprovided by the flap increases. For example, for a chip that operates atabout 35 degrees above ambient, the flap should provide a reduction of 5degrees, as opposed to a design without a flap. On the other hand, amore typical chip operating temperature is about 100 degrees aboveambient, for which the flap is expected to provide over 15 degreesreduction in maximum temperature.

Thus, while only certain embodiments of the invention have beenspecifically described herein, it will be apparent that numerousmodifications may be made thereto without departing from the spirit andscope of the invention. Further, certain terms have been usedinterchangeably merely to enhance the readability of the specificationand claims. It should be noted that this is not intended to lessen thegenerality of the terms used and they should not be construed torestrict the scope of the claims to the embodiments described therein.

1. A method for manufacturing a substrate for supporting an integratedcircuit chip thereupon, comprising: providing a sheet of stiffenercomprising a first conductive material; providing an insulating layer ondefined sections of the stiffener, each section defining a circuitryarea of one substrate; providing contacts on the insulating layer, thecontacts made of a second conductive material; cutting each substrateout of the sheet according to a designed outline, the designed outlinecomprising the circuitry area and a flap, the flap comprising a sectionof the sheet of stiffener having no insulating layer thereupon and acutout; and injecting adhesive into the cutout.
 2. The method of claim1, wherein the adhesive is provided between the integrated circuit chipand the flap.
 3. The method of claim 1, wherein the adhesive is a heatconducting epoxy.